Update libc headers to use musl headers.
Simplify some timezone code.

Since we only allow GMT as a timezone and ignore what the browser
says, we can simplify a lot of this code.
Fixup libcxx to work with musl includes.
Fix test_zlib. off_t is now 64bit.
Update to work with new libc headers.
Fix other.test_embind & other.test_static_link.
Fix time tests.
Add __va_copy as a compat/stdarg.h
Fix fpclassify with musl libc headers.

musl defines __fpclassify, __fpclassifyf, __fpclassifyl while
the old headers used __fpclassifyd for double rather than
__fpclassify.
Fix seekdir() for 64-bit off_t.
Fix ftello() for 64 bit off_t return value.
Provide better impl of __assert_fail().
Bump version for new libc headers.
Emscripten requires different #s for stdio fds.
Handle fpclassify and signbit for long double.